This invention relates to an image reading apparatus.
Conventionally known image reading apparatus are image reading apparatus of flatbed (FB) type and image reading apparatus of auto document feeder (ADF) type. The image reading apparatus of FB type reads a document by conveying in a sub-scanning direction a line sensor that can read image in a main scanning direction. The image reading apparatus of ADF type reads a document by conveying a document in a sub-scanning direction with a fixed line sensor. In these image reading apparatus, the document or the line sensor is conveyed so that a relative position between the document and the line sensor is changed at a constant speed. During the constant speed conveyance, the line sensor executes reading operation per predetermined time period so as to generate line image data which indicates a read image per line. Read image data indicating a read image of a document is generated by combining these line image data.
Among the above-described image reading apparatus, it is also known that there are image reading apparatus which transmit the line image data downstream after temporarily accumulating the line image data in a buffer. In such image reading apparatus, the buffer becomes full if there is a delay in processing downstream of the buffer in a data transmission path. Thus, in case that the buffer becomes or is likely to become full, document reading by constant speed conveyance is temporarily interrupted. The position where the document reading has been interrupted is stored in a memory. When a free space is created in the buffer, the document reading by the constant speed conveyance is restarted from the interrupted position.
The following technique is known in order to accurately restart the document reading from the interrupted position. For example, the known technique provides a line start signal which is synchronous with an encoder signal to the line sensor. The encoder signal is outputted from an encoder in accordance with a move of the line sensor. The line sensor executes the reading operation each time the line start signal is inputted, thereby to generate each of the line image data. If the line start signal is synchronous with the encoder signal, the interrupted position can be accurately specified from the encoder signal. Therefore, according to this technique, the document reading can be accurately restarted from the interrupted position. Furthermore, image degradation is inhibited which is due to difference between the interrupted position and the restart position.